Multipath fading occurs when a radio signal propagates along more than one path to the receiver. This phenomenon is caused by a variety of atmospheric conditions, such as temperature, humidity or air pressure gradients. In the majority of cases, the received signal during multipath fading can be represented as the result of signal propagation over a direct path and an indirect path. The signal delay associated with the indirect path is greater than that of the direct path. In communications systems, fading is undesirable because it can produce amplitude and/or delay distortion in the received signal. This distortion can be a major source of transmission deterioration which, under certain conditions, can exceed system performance objectives.
When we represent multipath fading by a two-path model, fading is characterized as being minimum or non-minimum phase fade. During a minimum phase fade, the amplitude of the signal propagating over the indirect path is less than the amplitude of the signal propagating over the direct path. Conversely, non-minimum phase fading occurs when the amplitude of the indirect path signal is greater than the amplitude of the direct path signal. It should, of course, be understood that this characterization of fading is a function of time and is basically unpredictable.
It has been found that automatic gain control as well as space diversity techniques are not adequate to maintain satisfactory transmission performance in the presence of fading. Recent efforts have centered on the use of equalization to mitigate fading effects and achieve the required level of system performance. Since fading is unpredictable, the compensation provided by a fade equalizer must be capable of automatically adapting for changing signal conditions. One such adaptive equalizer is disclosed by applicant in a U.S. Pat. application, Ser. No. 158,404, filed June 11, 1980, now U.S. Pat. No. 4,330,764 issued May 18, 1982 and assigned to applicant's assignee. This equalizer compensates for amplitude distortion as well as delay distortion during minimum phase fade conditions. However, it does not provide proper delay equalization for non-minimum phase fade. In fact, for non-minimum phase fade the equalizer doubles the delay distortion in the received signal. In another equalizer design disclosed by P. Karabinis in a U.S. Patent application, Ser. No. 293,463, filed Aug. 17, 1981 and assigned to applicant's assignee, the equalizer parameters are modified by the character of the multipath fading. Detecting the character of the fading, however, requires rather complex circuitry.